Multiple transmit and receive antennas have been proposed to provide both increased robustness and capacity in next generation Wireless Local Area Network (WLAN) systems. The increased robustness can be achieved through techniques that exploit the spatial diversity and additional gain introduced in a system with multiple antennas. The increased capacity can be achieved in multipath fading environments with bandwidth efficient Multiple Input Multiple Output (MIMO) techniques. A multiple antenna communication system increases the data rate in a given channel bandwidth by transmitting separate data streams on multiple transmit antennas. Each receiver receives a combination of these data streams on multiple receive antennas.
In order to properly receive the different data streams, receivers in a multiple antenna communication system must acquire the channel matrix through training. This is generally achieved by using a specific training symbol, or preamble, to perform synchronization and channel estimation. It is desirable for multiple antenna communication system to co-exist with legacy single antenna communications systems (typically referred to as Single Input Single Output (SISO) systems). Thus, a legacy (single antenna) communications system must be able to interpret the preambles that are transmitted by multiple antenna communication systems. Most legacy Wireless Local Area Network (WLAN) systems based upon OFDM modulation comply with either the IEEE 802.11a or IEEE 802.11g standards (hereinafter “IEEE 802.11a/g”). Generally, the preamble signal seen by the legacy device should allow for accurate synchronization and channel estimation for the part of the packet that the legacy device needs to understand. Previous MIMO preamble formats have reused the legacy training preamble to reduce the overhead and improve efficiency. Generally, the proposed MIMO preamble formats include the legacy legacy training preamble and additional long training symbols, such that the extended MIMO preamble format includes at least one long training symbol for each transmit antenna or spatial stream.
A number of frame formats have been proposed for evolving multiple antenna communication systems, such as MIMO-OFDM systems. Existing frame formats provide inaccurate estimations for the MIMO systems, such as inaccurate power measurement or outdated frequency offset and timing offset information, or fail to provide full backwards compatibility to the legacy devices of some vendors. In one proposed MIMO frame format, each transmit antenna sequentially transmits one or more long training symbols, such that only one transmit antenna is active at a time. As the transmit antennas are switched on and off, however, the temperature of the corresponding power amplifier will increase and decrease, respectively. Generally, such heating and cooling of the power amplifier will lead to “breathing” effects that cause the transmitted signal to have a phase or magnitude offset, relative to the desired signal.
It is therefore desirable to have a continuous transmission from all transmit antennas to avoid temperature related signal “breathing.” Thus, in further proposed MIMO frame formats, orthogonality is maintained using cyclic delay diversity (CDD) or tone-interleaving across different transmit antennas. The CDD short training symbol, however, cannot measure the received signal power with sufficient accuracy. Thus, additional backoff is required in the RF chain and additional dynamic range is required in the digitization process. Likewise, the tone interleaved design is not fully backwards compatible with a number of existing 802.11a/g devices that use short training for timing synchronization or use time domain channel estimation.
A need therefore exists for a method and system for performing channel estimation and training in a MIMO-OFDM system that is compatible with current IEEE 802.11a/g standard (SISO) systems, allowing MIMO-OFDM based WLAN systems to efficiently co-exist with SISO systems. A further need exists for MIMO preamble formats and training techniques that provide improved automatic gain control.